The development of biomass energy using renewable resources such as plants has been advanced as environmentally friendly energy resources replacing limited fossil fuel. For example, much starch is contained in edible portions of sugarcane, corn and the like, and high-purity ethyl alcohol, which can be used for automobile fuel or the like, can be comparatively efficiently produced from biomass thereof through steps such as saccharification, fermentation, distillation, purification. However, there is the problem of competition with food which mankind needs in biomass ethyl alcohol produced from food raw materials. When, the balance of CO2 emission in steps such as saccharification, fermentation, distillation and refining is considered also as to the balance of the CO2 emission for preventing warming, there has been the problem that it does not contribute so greatly.
Meanwhile, the development of technique for subjecting a microalga active in photosynthesis to self-fermentation to produce ethyl alcohol have also been advanced. According to this method, it is considered that sunlight energy can be converted efficiently, the steps of saccharification from starch and fermentation can be omitted, and it also contributes to an emission reduction in CO2. There is no problem of competition with food, either.
For example, Patent Literature 1 discloses a method for producing ethyl alcohol from a microalga, wherein the method relates to the technique of the self-fermentation of a microalga, a microalga which accumulates starch in cells is cultivated, slurry obtained by concentrating a culture solution containing the cultivated algal body is maintained in the darkness and an anaerobic atmosphere with the pH maintained in the range of 6.0 to 9.0 to produce ethyl alcohol.
For example, Patent Literature 2 discloses a new microalga which relates to a technique of the microalga which can be cultivated without needing a large amount of fresh water, grows at a salt concentration of sea water and accumulates starch in cells; produces ethyl alcohol from starch in cells by maintaining it under dark and anaerobic conditions; and belongs to Chlamydomonas sp.